The present invention relates to a method for etching an alloy containing iron and nickel as major components. More particularly, the present invention relates to a method for controlling a composition of an etching solution for micro-etching a shadow mask sheet of invar steel.
Conventionally, a mild steel plate has been used as a shadow mask mounted in a color cathode ray tube. In a shadow mask mounted in a color cathode ray tube which requires a high definition display and high resolution display, the diameter and pitch of the electron beam apertures must be made very small and with high precision. For example, in a shadow mask having circular apertures for passing the electron beams therethrough, the pitch is 0.2 mm, and the aperture diameter on the side of the electron gun is about 0.1 mm.
The most important function of a shadow mask for a color cathode ray tube is to transmit an electron beam through apertures of the shadow mask and to radiate correctly the electron beam onto predetermined positions of a phosphor screen. However, during operation the shadow mask expands due to heating by the electron beams radiated from the electron gun. As a result, the electron beam is not radiated correctly onto the predetermined positions of the phosphor screen, resulting in so-called mislanding, and hence, poor color purity.
This leads to a decisive drawback in the color cathode ray tube. Since invar steel has a very low linear expansion coefficient it may be used to solve the above problem. Invar steel is a nickel steel material and has a general composition of 0.2% or less of carbon, 0.5% manganese, 36% nickel and iron as the balance. Invar steel has a very low linear expansion coefficient of about 1.times.10.sup.-6 /deg at a temperature range of 0.degree. to 40.degree. C. This linear expansion coefficient is about one-tenth of that of mild steel.
An alloy such as invar steel having iron and nickel as its major components is generally etched using an aqueous solution of ferric chloride (FeCl.sub.3) as an etching solution. During the etching process, by the reaction of Ni and Fe the etching solution produces FeCl.sub.2 and NiCl.sub.2 and the like. However, since FeCl.sub.2 and NiCl.sub.2 do not have an etching ability, the etching capability is degraded as the amount of these materials in the etching solution increases. As a result, it is difficult to maintain the etching capability of the solution at a level required to perform etching. However, in the case of etching a mild steel plate, chloride gas is constantly dissolved, as indicated by formulae (1) below, so that ferrous chloride is oxidized to form ferric chloride and thus to allow recycling: EQU Fe+2FeCl.sub.3 .fwdarw.3FeCl.sub.3 EQU 3FeCl.sub.2 +(3/2)Cl.sub.2 .fwdarw.3FeCl.sub.3 ( 1)
Furthermore, by adding water to the etching solution, the etching solution can be controlled to have a predetermined etching capacity level.